<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>深入理解z-index</title>
  <style>
.box {
  position:absolute;
  background: blue;
  z-index:0;
} 
.box > img {
  position: relative;
  margin-left: -100px;
  z-index: -1;
}
  </style>
</head>
<body>

<div class="box">
  <img src="../images/54447b06000171a002560191.jpg">
</div>
  
</body>
</html>